Due to all the edits and clarifications asked I will re write the question tomorrow, thanks for your patience. Leaving this up until then, for reference.

In the setting I'm working on, magitech has created, among other things, a class of weapons called "projection blades".
When such a weapon is activated, it allows the cutting edge of the blade to interact with distant targets as if it were touching them.
Essentially you swing your sword at a training dummy 15 meters away and you cut it just as if you were right in front of it.

More details:

While the effect itself is invisible, as in, you only see a person swing a sword and something distant getting cut, you can visualize its dynamic by imagining that the effect is projected in a straight line parallel to the blade up to a certain distance (details pending, but for the purpose of this question let's say up to 50 meters, the exact length being adjustable ) and in that area it "bridges" the distance between the edge ofthe blade and whatever object ends in the way, for the purpose of touching and therefore cutting targets.
This effect doesn't have any other physical properties and therefore can't be used, for example, like a lever.
It's therefore optimal to trigger the effect at a specific point during a swing, as to avoid interaction with unwanted objects. This is possible thanks to a trigger on the handle.
To reiterate, the blade itself doesn't physically change in any way.

The effect moves from the blade to the target almost instantaneously, but this still means that an object in the way of what you want to cut will still partially or completely block your cut. Parrying and deflecting can therefore still be a thing, especially whith another projection blade.

The effect only transmits force, which is why you have to be moving relative to your target or swing in order to do damage. You can't just point the sword directly at your opponent and pull the trigger to impale him like with some kind of infinite length lightsaber.

Some energy is lost when you use your weapon this way, meaning you would hit harder with the actual blade rather than with the projected force effect. Obviously the convenience of turning a melee weapon into a ranged weapon at will more than makes up for it.

Force travels both ways, so your feel your weapon impacting with the target and you can potentially get it stuck, however this is quickly remedied by just turning it off.

Edit for additional details:

The "projection" can only be sustained for a very little time, details to be decided but let's say it's slightly below an entire second. This is more then enough time to strike and do damage.

After triggering the effect the magitech device needs to wait a moment before triggering again. Again, details pending but let's say 0.5 seconds. This seems like a small amount of lag but in martial arts and actual fighting I think this would make the weapon less "spammable" and encourage a more precise and deliberate fighting technique.

Given the dynamics and limits described above, what would be a plausible fencing style for users of this kind of weapons? Specifically for duels between projection weapon users.

Edit 2:

Forgot to mention that in this setting most armor and shields have been made obsolete by other kinds of weapons, creating something like the late 18th century in terms of tactics and equipment. This means that while armor and shields would probably be really effective against projection blades, they simply aren't around anymore and duels take place between unarmored opponents.

Edit 3 with yet more contextualization:

The weapons are used mainly due to their unique advantage scaling really well with user skill. If you are a good sharpshooter, you are still bound by musket-like weapons that can't fire more than a few shots per minute, but if you are a really good swordsman with a projection blade, you can defeat several unskilled opponents before they can even touch you, and be precise and quick enough that opponents armed with these early guns would actually find themselves at a disadvantage.
This means thar the weapon is seen on battlefields mostly as a status symbol for officers and decorated veterans, and in civilian use as an implicit statement "I'm using a weapon that requires more skill, beware". So duels happen more due to matters of honour or reputation, like cowboys in the far west and samurai in ancient Japan.
While there are surely more formalized contests between swordsmen, like academias and the like, the focus of the question should be on duelists of a more spontaneous kind.

Thank you and forgive me if the question is unclear in any way, this is my first post.

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

$\begingroup$Comments are not for extended discussion; this conversation has been moved to chat.$\endgroup$
– L.Dutch♦Apr 6 at 17:18

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$\begingroup$What have I told you about side effects? The newly introduced rules have the immediate effect that nobody will carry anything longer than a short knife. Due to the same law of levers, the sideways force which can be applied by the tip of a long sword is much smaller than the sideways force which can be applied by the tip of a short knife; since swordsmen can now transport the force at a distance, those armed with long swords will be at a massive disadvantage against those armed with short knives.$\endgroup$
– AlexPApr 6 at 17:37

$\begingroup$@AlexP ^ probably the only way to fix that (if it's something Geek wants fixed that is) is to say the distance the "magitech" can project a blades effects is somehow tied directly to the length of the blade itself, so one meter per inch of blade or some-such.$\endgroup$
– PelinoreApr 6 at 18:02

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$\begingroup$Hi and welcome. Posting questions is hard. I think yours would benefit from a re-write. There are too many edits (I think at this point they're just over half the question length) and it makes it a confusing read. Try the Sandbox perhaps?$\endgroup$
– CynApr 6 at 18:33

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$\begingroup$@StarfishPrime there is & you've only yourself to blame, if you hadn't played with that Ouija board that time I wouldn't be able to reach you from here ;D$\endgroup$
– PelinoreApr 6 at 20:30

5 Answers
5

Slashing: torque

In the 3rd century before the common era there lived in Sicily a Greek physicist and mathematician named Archimedes; his name literally means Master Mind¹, and he was indeed the greatest physicist and mathematician of the Antiquity.

¹) For the literalists: it actually means Great Planner or Great Thinker.

Among his greatest discoveries was the law of levers; namely, that the torques applied by the effort and the resistance must balance: hence his (attributed) quip that given a fulcrum and a long enough lever he could move the Earth.

Since the effect works as if the blade became very long, and it acts as a lever of the third class, if follows that the sideways force applied by the tip of the blade is minuscule, because the torque applied by the hand of the swords man must equal the torque applied by the resistance of the targets skin and clothes...

Thrusting: aim and tremor

Since slashing won't work, what about thrusting?

Well, here we run in the problem of aiming at the target. The blade is ten meters long, and the target is, say, 50 cm wide; thus the target subtends an angle of about 3 degrees. All right, a good swordsman should be able to aim his blade within 3 degrees. But wait! Not all the target is usefully hit. If the target is wearing any kind of armor then one must hit a specific area on the target, and aiming the tip of the sword at a small area ten meters away becomes a wonder in itself.

$\begingroup$As I said, the blade doesn't physically change, neither in length nor mass. The analogy of the blade increasing in length was more to describe how the range and shape of the effect was applied. You can swing it just fine, it's just that you draw a straight line from the blade to up to 50 meters away to detemine what actually feels the impact of the blade.$\endgroup$
– Inquisitive GeekApr 6 at 10:46

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$\begingroup$@InquisitiveGeek: And I explained that the torque applied by the hand of the swordsman equals the torque applied by the resistance of the target. The target is ten meters away; the mass of the blade is irrelevant, and I ignored it altogether. If the magic messes with mathematics then what's the purpose of asking a question? Anything is possible if mathematics itself can be altered.$\endgroup$
– AlexPApr 6 at 10:48

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$\begingroup$Maybe I didn't understand your response then, what I was trying to say is that this effect can't act like a lever, it's just an area of effect that allows you to ignore the distance between your target and the your blade, but other than that it has no other appreciable physical properties. It just "bridges" the gap between balde and point of impact for a moment. Also, I'm interested in how the weapon could be used by expert swordsmen fighting each other, not in how the weapon itself works, that is described at the best of my abilities in the main body of the question and is the basic premise.$\endgroup$
– Inquisitive GeekApr 6 at 10:56

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$\begingroup$@InquisitiveGeek: Then you must explain how it works, because it obviously doesn't work as if the blade became longer. Be careful with unintended side effects; while this answer shows that "it works as if the blade became longer" has potentially unwanted side effects, any rules must be carefully checked for such effects. For example, let's say that the effect works as if the space between the swordsman and the target was shrunk; this has the side effect that one can amplify torque by a factor of 20 without decreasing displacement: hey presto, a source of free energy!$\endgroup$
– AlexPApr 6 at 11:02

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$\begingroup$The blades used would shorten, because you don't need the length for reach anymore, and you won't need to waste torque and speed on long length. If you can reach 15 meters, you'd optimize for strength and speed of attack, with something just long enough to do serious damage. Slash 4 times with an oyster-knife versus once or twice with a more parry-able longer, heavier blade. Better A different leverage-ish problem is that you need to monitor/defend the whole 15m diameter area (706m^2) around you, it would be better to have allies close and gang up on targets with 2x-3x+ the attacks.$\endgroup$
– Dave XApr 6 at 17:13

Parrying Daggers and Bucklers

Given your responses in the comments, i offer you parrying daggers and bucklers. Compared to a ‘normal’ sized shield, these are far more convinient to use and carry.

Staring with the buckler, as it is the easiest to explain, this is a smaller dinner plate sized shield which is used far more actively than a larger shield. Rather than letting the blade come to you, you meet the blade with your shield. This could be a key defense against a blade which has such a long effective range, allowing you a moment to strike wih your own sword.

Parrying daggers are meant for catching and defecting sword strikes. Against a blade that only exists for about a second, they might not be all that useful as they are intended to bind a sword, allowing you to strike with your own.

However, if possible, the parrying dagger could contain technology similar similar to the projection blades but used in a different way. Rather than emitting a blade of their own, when a projection blade comes into contact with the dagger, rather than that blade only lasting a single second, the blade lasts for 2 or 3 seconds. This gives the user time to trap the projection blade using the dagger before stiking with their own projection blade. To counter this parrying dagger, you would have to only activate your blade after you have passed it, which could be hard to do against a skilled user.

You could also use a swordbreaker which has a similar use to a parrying dagger but uses teeth on the back edge rather than long, trident-like points. If you wanted to use the word differently though, your sword breaker might be the opposite of a parrying dagger. Rather than increasing the length of time the projection blade stays active, any projection blade it comes into contact with is instantly deactivated. A sword breaker would cancel out the effects of a parrying dagger, potentially allowing for some interesting fights if one duellist has a SB and the other has a PD. You could break a bind with a PD using your own SB, for example.

$\begingroup$I edited this to include the swordbreaker, it was an interesting idea that came to mind and i thought that the wordplay of it being used to actually break a sword was too good to resist.$\endgroup$
– Liam MorrisApr 6 at 13:31

$\begingroup$This is some really good perspective that I hadn't really thought about, but I'm I've been informed that I was too rash in accepting your answer so I'm withholding it for the sake of stimulating more discussion, though I find it very stimulating. Again sorry I'm just new here! In a few days I'll see which answer to accept as is custom here.$\endgroup$
– Inquisitive GeekApr 6 at 14:22

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$\begingroup$@InquisitiveGeek Dont worry about it, many new users do it and i fully expected the accepted tag to be withdrawn. As you were already told, its good practice to wait a minimum of 24 hours before accepting an answer but ideally you should wait a few days just to try and make sure people arent discouraged from answering. I think this might be on the help center but you’ll learn how things work around here soon enough. Also, top tip, when you hit 200 rep, join up with another SE website and you’ll get 100 points on this site, its called an association bonus.$\endgroup$
– Liam MorrisApr 6 at 14:35

Given the dynamics and limits described above, what would be a plausible fencing style for users of this kind of weapons? Specifically for duels between projection weapon users

One word:

FRENZIED

All sword fighting techniques are based on the idea of closing a gap to strike, then strikes and parries are exchanged until one of the combatants retreats a step or takes a hit.

What your weapons have done is essentially eliminated the option of moving out of striking distance. So once a duel starts thrusts, slashes and parries will be exchanged until one of the combatants drops.

$\begingroup$It has not completly eliminated the option to move out of striking distance, the weapons still have a fixed distance, its just that it is significantly longer than what a sword’s reach would normally be$\endgroup$
– Liam MorrisApr 6 at 13:40

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$\begingroup$@LiamMorris up close it's easy to judge the few inches difference between in range and out of range. That's not so easily done when 50 meters away.$\endgroup$
– K MoApr 6 at 13:48

$\begingroup$There is however the consideration of a .5 seconds between projections, so the exchange of blows can't be too frantic, otherwise one of the two opponents will end up unable to retaliate at a crucial moment, probably?$\endgroup$
– Inquisitive GeekApr 6 at 13:50

$\begingroup$The weapons act at range for a period of 1 second with a cool down of 0.5 seconds. It should be possible to defend while the sword is not activated, so a duelist at max could be on the attack for two thirds of the time.$\endgroup$
– K MoApr 6 at 14:11

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$\begingroup$@InquisitiveGeek Carry two weapons. Make a weird-ass two-pronged dagger where you can trigger the sword-gun feature on each blade alternately. Plenty of ways to add more frenzy to your frenzy.$\endgroup$
– Starfish PrimeApr 6 at 21:01

Sword and Shield

I would imagine the best fighting style in this scenario would be using an arming sword (your standard one-handed sword which has a blade about arms length) and some kind of large shield.

Given that this sword’s angle of attack can not be predicted in the same way a traditional sword’s can be, it necessitates the use of a large barrier to block the attack, a shield. The sword itself is inconsequential in this answer, you have already defined what it can do, all i can add is it should be a one-handed blade to allow the use of a shield.

The question then is what type of shield to use? There are several options available though i would say you want a large centre-grip (so it can be held out in front of your) shield that can cover a large area. This leaves us with three choices: the Kite shield, the Tower shield or a large Round shield.

I think you would be best of using a kite shield as it is the lightest of the three (allowing it to be moved and carried more easily) and it would adequately cover your body, even against the timed activation you mentioned. It is a large tear-drop shaped shield with the tip pointing down to your leg. If you wanted to reduce the risk of getting hit from unexpected angles, you may instead want a tower shield which is of a similar size but rectangular.

Given the recent edit has made most of this not relevant i propose an alternative option, the pavise. This is a rectanular shield that freely stands on the ground, traditionally it was used by crossbowmen to give them cover whilst they reloaded. If you are opposed to the use of traditional shields, this might be another option. You would also be able to use a two-handed weapon if that is what you wanted.

$\begingroup$Shields would be very effective, but as they would not be useful against anything else I feel like duelists probably wouldn't carry one around, considering how large and unwieldy they are. Still a good consideration that made me reflect on the worldbuilding involved, thank you!$\endgroup$
– Inquisitive GeekApr 6 at 12:35

$\begingroup$@InquisitiveGeek Well a further option could be the Buckler, a shield that is only the size of a dinner plate but can offer a huge amount of protection if one is skilled enough to use it. The buckler again is a centre-grip shield but is considerably smaller and lighter, and far more convinient to carry.$\endgroup$
– Liam MorrisApr 6 at 12:41

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$\begingroup$Bucklers could definitely be in, being small and portable.$\endgroup$
– Inquisitive GeekApr 6 at 12:50

I'm going out on a limb and stating that the less that a fighter has to worry about his reach, the more he has to worry about his perceptions of the enemy.

Imagine if you were to make a stabbing motion through your cloak or some other visual blocker that makes it very hard to predict where you were aiming. I imagine such a thing would be very difficult to parry.

The further away, and thus less clear your sight of the enemy, the greater the potential for bluff movements. To overcome this, I imagine people would prefer to use a large shield or an encompassing armor, assuming the funds and patience available for continually transporting such weighty objects.

So what we end up with are people either too impatient or too poor for lugging heavy and expensive equipment, and fight with knives and subtle misdirection/screen moves that are difficult to predict, and then another class of 'knight' type fighters who can afford the protection from that sort of combat but now need larger and harder-hitting weapons to go up against other 'knights'.

A long distance mace would do the trick there, but if you're trying to keep this purely to swordplay and/or protection is too effective or too-easily available it's possible that the enemies would have to close in on each other like normal sword combat to get around each other's shield/etc.